With the ever increasing importance of communication services, both speech and data, there is a rapidly growing need for providing economically cost efficient means of communication. The conventional public telephone service, usually known as public switched telephone network (PSTN) or wireline telephony, remains the predominant communication network used by millions of users around the world. Wireline telephony allows two users to communicate with each other only if there is a physical wire connection between the two users. The necessity of a physical connection between users imposes several restrictions on users of PSTN systems.
First, the capacity of users that a PSTN can service at a given time is restricted by the number of physical lines existing between users. Hence, the capacity of a PSTN can be increased only by installing/laying new lines to support additional users--a solution that is very expensive and time consuming. As a result, the capacity of a PSTN is usually capped by economic considerations. It would therefore be highly desirable to have a communication network whose capacity can be increased in a cost efficient manner.
Another problem with existing PSTN systems is the manner in which new lines are laid, usually below the ground. This involves extensive digging of roads and property. In many instances, this is not a viable option. This is especially true in populated urban areas like cities and suburbs where the demand for communication services is very high but there is very little land available for laying new PSTN lines. Furthermore, since PSTN lines stretch out over long distances between users, their maintenance is very expensive and time consuming. Thus, there is a need for a communication system which not only increases the communication capacity but is also easy to install and maintain.
The above-mentioned problems and restrictions of wired PSTN systems have a major impact on commercial establishments such as offices and hotels which require large communication capacity. A majority of these commercial establishments use private branch exchanges (PBXs) to handle their call processing needs. A PBX is generally a telephone exchange serving a single organization, for example, an office building having a switchboard and associated equipment, usually located on the user's premises. A PBX is generally connected to a plurality of private internal telephone users (called terminals) located in a typical office environment. The PBX provides for internal call switching between one or more of these terminals. The PBX is also connected to external telephone networks via PSTN lines. The PBX uses the PSTN lines to perform call routing between a PBX terminal and an external telephone network terminal.
As the commercial establishment served by the PBX grows in size, it frequently becomes desirable to increase the external communication capacity of the PBX at a reasonable expense. However, since the PBX is linked to fixed capacity physical PSTN lines, an increase in capacity is possible only by installing additional PSTN lines. As mentioned earlier, in most circumstances this option is prohibitively expensive and impractical. Consequently, the external calling capacity of these PBXs is forcibly limited to the number of existing PSTN physical links. It would therefore be desirable to provide a system for increasing the capacity of existing PBXs in a cost efficient and practical manner.
The advent of wireless telephony (or mobile communication) technology has sought to solve some of the restrictions posed by wired PSTN networks by decoupling the telephone from its wires. In a wireless communication network, for example GSM, the telephone is substituted by a mobile station (MS) or a handset. The geographical area serviced by the cellular network is divided in cells--each cell being served by a base transceiver station (BTS). Information is exchanged between the mobile station and the BTS over a radio interface. The BTS in turn communicates with a mobile services switching center (MSC) which performs the basic switching function in the wireless network. The MSC also acts as a gateway between the wireless network and other external communication networks--performing interconnectivity and switching functions between GSM and the other external telephone networks.
Although wireless telephony removes the restriction of wires, it has its own share of problems. First, a mobile station is quite expensive when compared to an ordinary telephone. Thus, it is not always economically feasible to replace a telephone with a mobile station. This is especially important for businesses and other commercial establishments which have existing PSTN telephone networks connected to PBXs. Replacing the entire existing PSTN base with a wireless network is not cost efficient. Second, many functions provided by the private branch exchange are not supported by wireless telephone services. It would therefore be desirable if the commercial establishments could retain their existing PSTN based PBX systems and also take advantage of the available wireless technology.